This invention relates to circuitry for use with magnetic bearings and, more particularly, to that for use with such bearings supporting a shaft rotating at high speeds.
The use of magnetic bearings to support a rotating shaft has found application in a number of different areas. One area is that of turbo-molecular vacuum pumps which include a shaft or rotor on one end of which is positioned an array of rotor blades which need to be rotated at high speed within the confines of a closely corresponding stator housing and blades. There is a great benefit in using magnetic levitation means as the rotor bearing.
In a known arrangement for driving levitation coils associated with such magnetic levitation means, an AC source of electric power is rectified to DC and supplied to a controller in which the source is rectified to a DC supply which powers both a DC motor drive linked to a DC motor of the pump (or whatever) and a regenerative power supply unit (PSU).
The output from the PSU is designed to power a linear amplifier whose output is passed outside the controller to magnetic levitation coils of a magnetic bearing of a vacuum pump (or whatever); the magnitude of the amount of power dram from the PSU and hence the output from the linear amplifier is determined by a feedback control signal to the amplifier from the bearing of a magnitude dependent on the degree of levitation afforded to the rotor so as to maintain the rotor at a predetermined axial position relative to the coils.
When the power supply is removed, rectifier means such as diodes, associated with the motor drive rectify the three-phase power generated by the DC motor which continues to power the regenerative power supply unit. However, with no power source, the output from the motor drive gradually drops to a minimum voltage of, say, 6 volts at which point the magnetic levitation ceases and "back-up" mechanical bearings take over to support the rotor.
In general, this allows a graceful collapse of the rotor on to the mechanical bearings when the AC power is mined off.
However, the operation of such an arrangement is not without its difficulties. For example, the amount of power associated with the regenerative PSU is limited owing to the facts that such units generate considerable quantities of heat which clearly increase with increasing power and that as a result the PSU has itself to be large in size.
In addition, the linear amplifier also has a limited current handling capability in view of the generation of heat therein, again leading to restrictions on the operational parameters of the arrangement as a whole.
Separately, any physical separation of the controller as a whole from the vacuum pump (or whatever) within which it is associated tends necessarily to lead to a substantially immediate collapse of the rotor on to its back-up mechanical bearings leading to possible mechanical shock/damage to the rotor and other components. Generally, such collapse can be tolerated only a few times prior to the pump being irretrievably damaged.
The present invention is concerned with a novel arrangement for powering levitation coils in such types of apparatus which generally allows such difficulties to be overcome.